Research on photoacoustic imaging apparatuses that acquire information about the inside of a subject by allowing light, such as a laser beam, emitted from a light source to enter and propagate through the subject has been actively carried out. In PTL 1, photoacoustic tomography (PAT) is proposed as such a photoacoustic imaging technique.
PAT is technique of visualizing information related to the optical characteristic of the inside of an organism, which is a subject, by irradiating the organism (subject) with pulsed light emitted from a light source, receiving an acoustic wave generated when the light that has propagated and diffused through the subject is absorbed by the organism's tissue, and analytically processing the received acoustic wave. In this way, information about biological information, such as an optical-characteristic-value distribution in the subject, and, particularly, an optical-energy-absorption density distribution can be acquired.
In PAT, an initial acoustic pressure P0 of an acoustic wave generated from an optical absorber inside the subject can be represented by the following expression.P0=Γ·μa·Φ  (1)
Here, Γ represents the Grüneisen coefficient and is obtained by dividing the product of the isobaric volume expansion coefficient β and the square of sonic speed c with isobaric specific heat CP. Γ is known to be a substantially constant value when the subject is specified, where μa represents an optical absorption coefficient of an absorber, and Φ represents the light intensity (which is the intensity of light incident on the absorber and is also referred to as optical fluence) in a local area.
The change over time of the acoustic pressure P, which is the volume of an acoustic wave propagated through the subject is measured, and an initial-acoustic-pressure distribution is calculated from the measured result. By dividing the calculated initial-acoustic-pressure distribution with the Grüneisen coefficient Γ, the distribution of the product of μa and Φ, i.e., the optical-energy-absorption density distribution, can be acquired.
As represented by Expression 1, to acquire the distribution of the optical absorption coefficient μa from the distribution of the initial-acoustic-pressure distribution, it is necessary to determine the distribution of the light intensity Φ inside the subject. When an area sufficiently large with respect to the thickness of the subject is irradiated with a uniform amount of light, the distribution Φ of the light intensity in the subject can be presented by the following expression when light propagates through the subject as plane waves.Φ=Φ0·exp(−μeff˜d)  (2)
Here, μeff represents an average effective attenuation coefficient of the subject, and Φ0 represents the amount of light incident on the subject from a light source (the light intensity at the surface of the subject). Furthermore, d represents the distance between the area on the surface of the subject irradiated with the light emitted from the light source (light irradiation area) and the optical absorber in the subject.
By using the light intensity distribution Φ represented by Expression 2, the optical absorption-coefficient distribution (μa) can be calculated from the optical-energy-absorption density distribution (μaΦ) of Expression 1.